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Magic Stripes
COLOR by Polarized Transmission (Polariscope)
Optics Suitcase

Remove the Slinky from the suitcase and choose a volunteer to come up front.

  • Give the student one end of the slinky and ask her (him) to hold this end steady at mid-chest level. Stand 4 feet away and begin to vibrate your end up and down and in a circle. You should be able to create a standing wave with a few nodes, but the plane of vibration should not be well defined.

  • State that, in addition to color, light has a wave nature. The slinky represents a light wave. This motion represents unpolarized light - light without a preferred vibration direction. [For simplicity, we ignore circularly polarized light.]

  • Stop the circular motion and vibrate only vertically. State that light is “polarized” when it vibrates in one direction - vertical or (switch hand motion) horizontal. [Horizontal motion is a bit harder to maintain while speaking and you might go back to the vertical motion]

  • Define linearly polarized light as light whose vibration direction is in a plane. [Here you might use the overhead transparency from NASA that shows the spectrum from radio to gamma-rays, with low frequency, long wavelength and high frequency, short wavelength radition. The photos at the bottom of the transparency show what the Milky Way looks like through different telescopes that “look” at differ- ent frequencies or wavelengths of optical radiation.]

  • Put the slinky away.

Have the teacher hand out the Magic Stripes Theme Packet, but ask the students not to open them yet. When everyone has their own packet, proceed as follows:

  • Ask the students to remove everything from the packet and place the 5 items on their workspace. (See Figure 3.) Have them check that they have two dark pieces of plastic, one transparent piece of plastic with writing on it, a fork, and a plastic vial.
  • Take your two large pieces of linear sheet polarizer and hold them up, one in each hand. Combine them about one foot in front of your face with the transmission axes parallel. You should be able to see the students, and they should be able to see you. Ask them to make a sandwich in the same way with their dark pieces of plastic.
  • During the “oohs” and “ahs”, reveal that each dark plastic has a secret code on its surface in the form of small lines. Combining the plastics so that the lines are parallel makes it possible to see though them. Combining with lines perpendicular, or “crossed”, blocks the light. Identify the plastics as linear polarizers. One plastic held up to the room lights causes the unpolarized white light to become linearly polarized. Once the light is linearly polarized it vibrates in one plane, and it is either transmitted or absorbed by the second plastic polarizer. Polarized sunglasses are made of this plastic.
  • Have the teacher turn off the lights. Place the large polarizers on the over head projector and demonstrate how they polarize and extinguish the white light from the projector. Separate the crossed polarizers at four corners with the plastic cups to make a polariscope, place the plasticware inside, stand back, and enjoy the excitement.
  • Ask where the color comes from. [You might get some correct answers.]
  • Explain that stresses inside of transparent materials degrade the quality of linearly polarized light coming through the polariscope, causing various colors to show up. With a polariscope, geologists identify certain crystals and mineral structures. Civil engineers examine stresses inside structures made out of transparent plastic, to understand how to build them better. Photonics technicians evaluate the quality of laser glasses and laser crystals with polariscopes.
  • Have the room lights turned back on. With the items from your packet, show the students how to make a polariscope in one hand. Ask them to find the colored stripes in the clear sheet of plastic from their packet. [While looking through the crossed polarizer sandwich at the overhead lights, they must insert the clear plastic between the polarizers.]
  • Ask the students to evaluate the internal stresses in the plastic vial and the fork. By squeezing on the tines of the fork, the students may be able to induce and visualize additional stresses.
  • Have everyone put all items back in the packets. Suggest that, once home, the students may demonstrate the magic stripes trick to their families, since they know the secret polarizer code and how to construct a polariscope.

Magic Strips

Figure 3. Contents of the Magic Stripes Theme Packet, including supplies for making a polariscope.

Copyright by Stephen D. Jacobs, Rebecca L. Coppens and Christine Andrews-Angelo
December 24, 2001

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